The process for recovery of elemental metal values from ores and processing liquids by solvent extraction-electrowinning (hereafter, "SX-EW") is well-known. Briefly, the process is carried out using a metal-bearing aqueous solution. Such metal-bearing solution is obtained by dissolving (generally from an ore) the desired metal in an aqueous leach liquor, or by using a metal-bearing solution such as process effluent. The resulting solution of metal values is mixed with a water-immiscible organic solvent (e.g., kerosene) containing a water-insoluble ion exchange composition having selective affinity for the desired metal values. The ion exchange composition preferentially extracts the desired metal values from the aqueous solution. The aqueous and organic phases are separated. The aqueous solution, now metal-depleted, is usually referred to as "raffinate". The raffinate can be recycled as leach liquor (in a leaching process) or discarded (in a process such as recovery of metal from process effluent). The organic phase (which contains ion exchange composition and the extracted metal values) is usually referred to as "loaded organic". The desired metal values are removed from the loaded organic by mixing with an aqueous strip solution containing strong acid such as sulfuric, phosphoric, or perchloric acid, and having lower pH than the above metal-bearing aqueous solution. The aqueous strip solution extracts the desired metal values into the aqueous phase. After separation of the organic and aqueous phases, the desired metal values are present in the aqueous strip solution, and the resulting metal-enriched strip solution is usually referred to as "electrolyte" or "pregnant electrolyte". The metal-depleted organic phase is usually referred to as "spent organic". Such spent organic can be recycled for fresh loading with metal values by mixing with metal-bearing aqueous solution. Metal isolation as described above is generally referred to as "solvent extraction" (hereafter, "SX"). The desired metal is recovered in purified form by electroplating the metal from the electrolyte. Such recovery by electroplating is generally referred to as "electrowinning" (hereafter "EW"). After recovery of the desired metal, the metal-depleted electrolyte is usually referred to as "spent electrolyte". Such spent electrolyte can be recycled as aqueous strip solution for fresh loading with metal values by mixing with loaded organic.
The SX-EW process is carried out commercially on a continuous basis and is used for the recovery of metals such as copper or nickel. Industrial use of the SX-EW process is increasing due to its efficiency, low energy costs, low pollution levels, and simplified materials handling requirements. The SX-EW process is described, for example, in Tuddenham, W. M. and Dougall, P. A., "Copper", Kirk Othmer Encyclopedia of Chemical Technology, 3rd Ed., Vol. 6, 850-852 (1979), McGarr, H. J., "Solvent Extraction Stars in Making Ultrapure Copper", Chemical Engineering, Vol. 77, No. 17, Aug. 10, 1970, pp. 82-84, and Merigold, C. R. and House, J. E., "The Application of Liquid Ion Exchange Technology to the Recovery of Copper" (a paper presented at the Copper Technology Seminar in Washington, D.C., December 1975). Flow charts showing the SX-EW process are included, for example, in Tuddenham et al, id at 851 and in McGarr, id at 83-84.
During the electrowinning step, elemental metal is plated out at the electrowinning cathode and oxygen evolves at an insoluble anode. The evolution of oxygen gas entrains strong acid electrolyte, carrying it into the air above the electrowinning tank in the form of a fine mist or spray. This mist or spray then spreads throughout the electrowinning tankhouse. The acidic mist is corrosive and a health hazard and can cause extreme discomfort to the skin, eyes, and respiratory systems of tankhouse workers, especially during hot weather conditions. This has caused high turnover among tankhouse workers.
A similar mist-formation problem once occurred in the chromium plating industry. Chromium plating companies employed extensive ventilation above plating tanks, clothed workers in heavy protective garments, and floated plastic balls on the surface of the electrolyte to reduce mist-formation and problems caused by such mist. These expedients were cumbersome and insufficiently effective. The use of such expedients was made unnecessary after the discovery and use of certain stable fluorochemical surfactants which, when added to a chromium plating bath, promoted formation of a foam at the surface of the plating bath which effectively eliminated chromic acid mist formation. Such fluorochemical surfactants are described, for example, in U.S. Pat. Nos. 2,750,334, 2,750,335, 2,750,336, and 2,750,337.
As described above, the SX-EW process is generally carried out on a continuous basis, with recycling and regeneration of the metal-bearing aqueous solution, the organic phase, and the electrolyte. Thus, after a portion of the desired metal has been plated from the electrolyte, the spent electrolyte is mixed with fresh loaded organic. This process subjects the electrolyte to a series of stages in which the electrolyte is mixed with loaded organic, phase separated, subjected to electroplating conditions, and recycled. Certain fluorochemical foam-forming surfactants such as those commonly used in the chromium plating industry proved to be unsatisfactory for inhibiting acidic mist formation above electrowinning tanks used in the SX-EW process. For example, the conventional chrome plating fluorochemical mist suppressant C.sub.8 F.sub.17 SO.sub.3 K gave good initial foam formation and mist suppression above a copper electrowinning tank, but the fluorochemical was rapidly extracted into the organic phase during recycling of the electrolyte, and subsequently was extracted into the raffinate. In addition, the fluorochemical surfactant C.sub.8 F.sub.17 SO.sub.3 K was found to interfere with copper recovery and to retard phase separation between organic and aqueous phases when used with ion exchange compounds such as "Acorga P5300" (commercially available from Imperial Chemical Industries, Ltd.) and "LIX 64N" (commercially available from Henkel Corporation).
In order to suppress acidic mist formation in the electrowinning tankhouse, SX-EW metal producers have utilized mist suppression expedients such as those used in the chrome plating industry, before discovery of suitable foam-forming mist suppressing agents. For example, SX-EW producers employ extensive ventilation in the electrowinning tankhouse, clothe workers in protective garments, and float plastic balls on the surface of the electrowinning electrolyte. These means are cumbersome and only partially effective, especially during hot weather. Also, electrowinning tanks have been covered with polypropylene tank blankets, and in U.S. Pat. No. 3,948,747 there is described a mist suppressing means for copper SX-EW carried out by floating elongated members (such as plastic rods) on the electrowinning electrolyte.